This invention relates generally to photovoltaic devices. More specifically, the invention relates to photovoltaic devices of the P-I-N type. Most specifically, the invention relates to a high deposition rate process for the preparation of P-I-N type photovoltaic devices, which devices have performance characteristics which are at least equal to the performance characteristics of comparable devices made at lower deposition rates.
Photovoltaic devices provide clean, quiet and reliable sources of electrical power. The earliest photovoltaic devices were manufactured from single crystalline material. These devices were expensive, delicate, relatively bulky and difficult to manufacture in large area configurations. Various techniques have now been developed for preparing thin film semiconductor materials which manifest electrical properties which are equivalent, and in many instances, superior, to their crystalline counterparts. Such thin film materials may be readily deposited over very large areas and on a variety of substrates. Such alloys and techniques for their preparation are disclosed, for example, in U.S. Pat. Nos. 4,226,898 and 4,217,374. One important class of photovoltaic devices comprises a layer of substantially intrinsic semiconductor material interposed between two oppositely doped semiconductor layers. Such devices are termed P-I-N or N-I-P devices depending on the order of the layers, and these two terms shall be used interchangeably herein. In many instances, a series of such P-I-N devices may be stacked in tandem so as to be in a series electrical and optical relationship.
Thin film semiconductor materials may be prepared by a variety of techniques, and vapor deposition techniques are one particularly preferred class of deposition processes. In a typical vapor deposition technique, a process gas, which includes a precursor of the semiconductor material, is introduced into a deposition apparatus, most typically at reduced pressures. The process gas typically comprises a mixture of materials. In the apparatus, the process gas is subjected to activating energy which decomposes the process gas into deposition species, and these species are contacted with a substrate upon which they deposit a layer of semiconductor material. Typically, the substrate is maintained at an elevated temperature so as to facilitate the deposition. By varying the process gas and deposition conditions, succeeding layers of semiconductor material of various compositions and types may be deposited in sequence upon a substrate so as to create various configurations and devices. The activating energy employed to create the deposition species may comprise one or more of thermal energy, electromagnetic energy (including direct current, radio frequency energy and microwave energy) as well as photon energy or thermal energy. Deposition techniques of this general type are well known in the art, and while the present invention will be primarily described with reference to electromagnetically energized plasma deposition processes, it is to be understood that the present invention may be utilized in connection with other such thin film deposition processes.
It is conventional wisdom in the photovoltaic arts that photovoltaic devices manufactured in a high rate deposition process have performance characteristics, as measured by parameters such as fill factor and efficiency, which are lower than are those of comparable devices manufactured in a lower rate deposition process. Clearly, economics favors the use of high speed deposition processes for the large scale manufacture of photovoltaic devices. At the same time, it is also desirable that such devices have high efficiencies. Heretofore, the parameters of high deposition rate and high efficiency have been mutually exclusive.
In accord with the present invention, it has been found that through the control of deposition rates, photovoltaic devices can be manufactured at an overall high average rate of deposition and still manifest performance characteristics which are equal to, and in some instances superior to, those of comparable devices manufactured through the use of a lower rate process.
There is disclosed herein a method for the manufacture of a P-I-N type semiconductor device which is comprised of a body of substantially intrinsic semiconductor material disposed between a body of P type semiconductor material and a body of N type semiconductor material. In a first step of the method, there is provided a first, doped body of semiconductor material of a first conductivity type. A deposition apparatus, having a deposition region defined therein, is provided, and this process gas is subjected to activating energy which decomposes it into deposition species. The first, doped body of semiconductor material is contacted with said deposition species in said deposition region so that the deposition species deposit a body of substantially intrinsic semiconductor material onto the first, doped body at an average deposition rate of N. A second, doped body of semiconductor material of a second conductivity type, opposite the first conductivity type, is deposited upon the body of substantially intrinsic material so that the second, doped body of semiconductor material is separated from the first, doped body of semiconductor material by the body of substantially intrinsic semiconductor material. In accord with the present invention, the rate at which the deposition species deposit the body of substantially intrinsic material onto the first, doped body is controlled so that a portion of the body of substantially intrinsic semiconductor material which is disposed closest to the interface with the P type semiconductor material, and which comprises at least 10% of the thickness of the body of substantially intrinsic semiconductor material, is deposited at a deposition rate which is less than N.
In one embodiment of the invention, the low deposition rate portion of the body of substantially intrinsic semiconductor material comprises at least 20% of the thickness of the body of substantially intrinsic semiconductor material. In another embodiment of the invention, the low deposition rate portion comprises at least 30% of the thickness of the body, and in yet another embodiment, the low deposition rate portion comprises no more than 50% of the thickness of the body.
The rate of deposition may be controlled by controlling the intensity of activating energy to which the process gas is subjected. In other embodiments, the deposition rate is controlled by controlling the composition and/or pressure of the process gas, and in yet other embodiments, the deposition rate is controlled by controlling the access of the deposition species to the first, doped body of semiconductor material, or by controlling substrate temperature.
The activating energy may comprise electromagnetic energy, which may be direct or alternating current energy. In some embodiments, the activating energy is radio frequency energy, and in other embodiments, the activating energy is microwave energy. In yet other embodiments, the activating energy may comprise photon energy.